To be quite honest, If you've never used an underwater camera before. Forget the underwater strobe to start with. It will add too much task loading to your diving. Even a fully auto TTL strobe will necessitate aiming etc.
I started playing about 2 years ago with my old Olympus C-830L (D-330 statestide I think) in an Ike housing. Camera was totally point and shoot no manual settings. I was thrilled with the pictures taken. Eventually I outgrew the set and added a strobe later and am now waiting to house my 5060.
You can take excellent pictures with the internal flash, maybe not to pro/semi-pro standard, but good nonetheless. Remember to force the flash (you should see the lightning bolt on the camera display continuously) on in all conditions so it fires even in bright light. Try to get close to the subject you are taking, or stick to sillhouetes. TO get started try sticking to closeups using Macro mode which should give you the best results initially. I did have some success in the Red Sea & Thailand using a color correcting filter such as those sold by UR-Pro. You need the orange one for blue water. You need bright light for best results and the colour correction is a compromise (the actual level of compensation required depends on depth), but it gave me nice results and put off a strobe purchase for a little longer. I got best results using a filter by disabling internal flash.
The DS-50 is a good choice and I think more powerful than the FL-20. Though the FL-20 can be used topside. If you're using the PT-20 the DS-50 will not be hard wired, you'll need an additional optical sensor that reads the cameras internal flash output either TTL or Manual. Manual is reported to give best results, but if you shoot in P mode go TTL. Don't forget to consider the cost of strobe arms & tray as well. You need something to mount the strobe on.
All of the other advice is valid, but I would say start easy and don't try and do too much. On the other hand if Bonaire is the trip of a lifetime and you want the greatest shots, tool up with the best strobe or strobes you can afford, learn manual mode, and get loads of practice dives in with the new kit.
Otherwise, for snapshots (people will hate me for saying this) just point & shoot in P mode! If you can afford it, try a filter. It's hassle free and requires no extra brainpower to operate. I used to take it on/off during the dive to alternate between wide angle shots using no flash and closeup Macros with flash.
Above all, don't let camera & kit get in the way of enjoying the diving!